1. Field of the Invention
The present invention relates generally to an apparatus and method for cooperative relaying based on beamforming in a mobile communication system, and in particular, to an apparatus and a method for performing beamforming in consideration of a decoding error of a relay station.
2. Description of the Related Art
Extensive research is being conducted on a cooperative communication scheme for improving performance of a mobile communication system. In the cooperative communication scheme, a Mobile Station (MS) and a Base Station (BS) communicate in cooperation with a plurality of Relay Stations (RSs), another MS, or other BSs, thereby improving link efficiency.
FIG. 1 is a diagram illustrating a conventional beamforming-based cooperative relay system.
Referring to FIG. 1, a BS 100 with a single antenna transmits a signal to a plurality of RSs 110 adjacent to an MS 120 and the RSs 110 operate as a virtual multiple antenna device to retransmit the signal to the MS 120. The cooperative communication scheme can achieve a multiple-antenna gain in a single-antenna environment, and can achieve a greater gain when the link condition between the BS and the MS at a cell boundary is poor.
In the cooperative communication scheme, the RSs 110 serve as a virtual signal transmission group to transmit a signal that is received from the BS 100, to the MS 120 and transmits a signal that is received from the MS 120, to the BS 100. Herein, for DownLink (DL) transmission, the RSs 110 independently decode a signal received from the BS 100, encode the decoded signal, multiply the encoded signal by a beamforming weight Wk and simultaneously retransmit the resulting signal to the MS 120. The MS 120 decodes the received beamformed signal. When the RSs 110 retransmit a signal by beamforming, higher link efficiency is achieved than where the BS 100 transmits a signal directly to the MS 120. That is, by the RSs 110 completely decoding a signal received from the BS 110 and retransmitting the decoded signal to the MS 120 without an error, higher performance is achieved.
However, if one or more of the RSs 110 fail to decode the received signal, e.g., if the RSs 110 re-encode a signal with a decoding error and transmit the re-encoded signal to the MS 120, the decoding performance of the MS 120 may degrade. In order to solve this problem, a scheme has been proposed in which the RS 110 with a decoding error re-receives the signal from the BS 100 until it succeeds in decoding the received signal. However, while this scheme enables the RS 110 to completely decode the signal, it also increases the transmission delay and the resource consumption.